Molecular sieve zeotypes are a commercially important class of materials that have distinct crystal structures with defined pore structures that are shown by distinct X-ray diffraction (XRD) patterns and have specific chemical compositions. The crystal structure defines cavities and pores that are characteristic of the specific type of molecular sieve zeotype.
Molecular sieves have numerous industrial applications, and zeolites of certain frameworks, such as CHA, are known to be effective catalysts for treating combustion exhaust gas in industrial applications including internal combustion engines, gas turbines, coal-fired power plants, and the like. In one example, nitrogen oxides (NOx) in the exhaust gas can be controlled through a so-called selective catalytic reduction (SCR) process whereby NOx compounds in the exhaust gas are contacted with a reducing agent in the presence of a zeolite catalyst.
Two types of molecular sieve zeotypes are aluminosilicate zeolites and silicoaluminophosphates (SAPOs). Zeolites were traditional considered to be crystalline or quasi-crystalline aluminosilicates constructed of repeating TO4 tetrahedral units with T being most commonly Si and Al (although other T-atoms and combinations of T-atoms are known). These units are linked together to form frameworks having regular intra-crystalline cavities and/or channels of molecular dimensions. Aluminosilicate zeolites can also be distinguished on the basis of their chemical composition such that zeolites X and Y are both FAU topological types but have different silica-alumina ratios (different SARs). Similarly, SSZ-13 has the CHA topological type but has a higher SAR than previous CHA topological types. Such differences in chemical composition reflect profound differences in both the preparative conditions and the properties and applications of the resulting material.
SAPO's have a three-dimensional microporous crystalline framework made of PO2+, AlO2−, and SiO2 tetrahedral units. Because an aluminophosphate (AlPO4) framework is inherently neutral, the incorporation of silicon into the AIPO4 framework by substitution generates a charge imbalance and this can lead to these materials having acidity. Controlling the quantity and location of silicon atoms incorporated into an AIPO4 framework is important in determining the catalytic properties of a particular SAPO molecular sieve, such that as with aluminosilicate zeolites the precise chemical composition of the framework is important in defining the properties and applications.
Numerous types of synthetic zeolites have been synthesized and each has a unique framework based on the specific arrangement its tetrahedral units. By convention, each topological type is assigned a unique three-letter code (e.g., “GME”) by the International Zeolite Association (IZA).
The catalytic properties of both aluminosilicate zeolites and SAPO materials can be modified after the aluminosilicate zeolite and/or SAPO molecular sieve has been synthesized. This type of “post-synthesis” modification can be accomplished by treating the (usually calcined form) of the molecular sieve with metallic, semi-metallic or non-metallic materials comprising nickel, cobalt, manganese, magnesium, barium, strontium, lanthanides, actinides, fluorine, chlorine, chelating agents, and others. The modifiers may or may not become part of the final composition of the modified catalyst.
Synthetic zeolites of the SFW topological type when prepared as aluminosilicate compositions are produced using structure directing agents (SDAs), also referred to as a “templates” or “templating agents”. The SDAs that are used in the preparation of aluminosilicate SFW topological type materials are typically complex organic molecules which guide or direct the molecular shape and pattern of the zeolite's framework. Generally, the SDA can be considered as a mold around which the zeolite crystals form. After the crystals are formed, the SDA can be removed from the interior structure of the crystals, leaving a molecularly porous aluminosilicate cage.
In typical synthesis techniques, solid zeolite crystals precipitate from a reaction mixture which contains the framework reactants (e.g., a source of silica and a source of alumina), a source of hydroxide ions (e.g., NaOH), and an SDA. Such synthesis techniques usually take several days (depending on factors such as crystallization temperature) to achieve the desired crystallization. When crystallization is complete, the solid precipitate containing the zeolite crystals can be separated from the mother liquor which can be discarded. This discarded mother liquor contains unused SDA, which is often degraded due to harsh reaction conditions, and unreacted silica.
The SFW framework has been recently discovered and synthesised as an aluminosilicate (SSZ-52) by Zou and coworkers (D. Xie, L. B. McCusker, C. Baerlocher, S. I. Zones, W. Wan, X. Zou, J. Am. Chem. Soc., 2013, 135, 10519-10524) The use of a polycyclic quaternary ammonium cation (N,N-diethyl-5,8-dimethyl-2-azonium bicyclo[3.2.2]nonane) as SDA leads to the formation of a stacking-faulted structure. More recently, alternative SDAs (N-ethyl-N(2,4,4-trimethylcyclopentyl)pyrrolidinium or N-ethyl-N-(3,3,5-trimethylcyclohexyl)pyrrolidinium) have been designed and used to prepare SSZ-52. However, the reported PXRD patterns indicate considerable faulting (T. M. Davis, A. T. Liu, C. M. Lew, D. Xie, A. I. Benin, S. Elomari, S. I. Zones, M. W. Deem, Chem. Mater., 2016, 28(3), 708-711).
There is a need to develop new molecular sieves having the basic structure of known molecular sieves, where minor changes in the structure can affect one or more of the catalytic properties of the molecular sieve. In some cases, while minor changes in the structure may not be discernable using normally used analytical techniques, the catalytic activity of the structurally modified molecular sieves may be improved relative to very closely related analogous molecular sieves. Unexpected improvements in the catalytic activity of such structurally modified molecular sieves can allow for the compositions of exhaust gases from engines to meet various regulatory requirements.